The present invention relates to a protection system of a powerplant; and more particularly to a method and system for automated overspeed protection on a powerplant comprising multiple powerplant machines integrated by at least one shaft.
Some powerplants are configured with multiple powerplant machines, such as, but not limiting of, turbomachines, and others with rotating shafts. The powerplant configurations may include, but are not limited to, a steam turbine and a gas turbine integrated by a single shaft (commonly called a single-shaft STAG); or a multiple turbines integrated via multiple shafts.
An overspeed condition occurs after the speed of a shaft on a powerplant machine exceeds a specified range. During the overspeed condition, a powerplant machine typically experiences severe mechanical and thermal stresses that can cause a catastrophic failure.
Generally, the powerplant machine is equipped with protection systems, which attempt to reduce the effects of an overspeed condition. A governor system generally serves as the primary line of protection. Upon detecting an overspeed condition, the governor attempts to decrease the speed of the shaft. There may also be a secondary, or independent line, of protection—an overspeed protection system. Typically, the overspeed protection system incorporates mechanical, electrical, and software components to safeguard the powerplant machine. An overspeed protection system protects the powerplant machine by initiating an emergency shutdown (commonly called a trip) during an overspeed event.
Powerplant operators periodically test the overspeed protection system to determine if the system is functioning properly. Prior to testing the overspeed protection system, the powerplant machine is customarily operating in a full-speed-no-load (FSNL) condition. FSNL is a condition when the powerplant machine is at a normal operating speed and is not exporting energy to a load. An overspeed test typically involves manually raising the speed of a shaft above the normal operating range. For example, but not limiting of, during an overspeed test operators may raise the speed of the shaft to 110% of the normal operating speed; thereafter the overspeed protection system should trip the powerplant machine.
There are a few problems with the current method of overspeed testing. Manually adjusting the shaft speed may introduce high thermal transients. A trip at a speed near or above the normal operating speed can introduce large mechanical, electrical, and thermal stresses on the components of the powerplant machine. These stresses decrease the maintenance interval. Moreover, after a trip, a re-start of the powerplant machine is required, which delays the exporting of energy. In addition, the current overspeed testing methods typically require that the powerplant machine operate at FSNL. This does not generate revenue but consumes fuel and electricity. These problems drive powerplant machine operators to avoid manual speed adjustments, trips, FSNL operation, and overspeed testing.
For the foregoing reasons, there is a need for a method of testing an overspeed protection system that reduces the chance of a trip. The method should be adaptable to a wide variety powerplants, such as, but not limiting of, a single shaft STAG.